Optical filters used for purposes of gain flattening produce complex spectral responses that adjust relative optical power among a plurality of different wavelength channels. Interference filters formed from thin layers of optical materials differing in refractive index, which have been previously used for providing much simpler spectral responses, are subject to additional design considerations that expand spectral response capabilities while employing precise manufacturing controls.
Thin-film interference filters having relatively simple band-pass or band-reject spectral responses can be constructed by depositing alternating layers of two different dielectric materials on a transparent substrate. The dielectric materials differ in refractive index. One of the dielectric materials is distinguished as a high index material, and the other dielectric material is distinguished as a low index material.
A manufacturing technique referred to as xe2x80x9cturning point monitoringxe2x80x9d requires each of the alternating layers to have a physical thickness equal to an integer multiple of a one-quarter-wavelength travel of a narrow-band monitoring light beam as transmitted through the layers. The alternating layers have the same or an integer multiple of the same optical thickness but have physical thicknesses that differ because of differing refractive indices. The quarter-wavelength travel of the monitoring beam (i.e., the physical thickness) is calculated as one-quarter of the wavelength of the monitoring beam in a vacuum divided by the refractive index of the layer material in which the beam is transmitted. The wavelength of the monitoring light beam is generally close to the central wavelength of the interference filter.
As each layer is vacuum deposited, multiple reflections of the monitoring light beam propagating through the deposited layers produce interference effects that vary between points of maximum and minimum interference (i.e., local transmission extrema) at quarter-wavelength thicknesses of the layers. Deposition is switched at turning points from one of the high or low index materials to the other as the appropriate transmission extrema are reached.
The xe2x80x9cturning point monitoringxe2x80x9d technique has an inherent self-correcting effect that reduces cumulative errors as well as the collective effects of purely random errors. Larger individual layer thickness errors can be accommodated than those of conventional thickness measuring techniques, such as from timed deposits, because each transmission extremum is based in part on the optical performance of all of the preceding layers in addition to the layer whose thickness is being monitored. The benefits of xe2x80x9cturning point monitoringxe2x80x9d as a self-correcting manufacturing technique are discussed in and article entitled xe2x80x9cTurning point monitoring of narrow-band all-dielectric thin-film optical filtersxe2x80x9d, by H. A. Macleod in Optica Acta, Volume 19, Number 1, 1972.
More precise band filter responses, such as for reducing pass-band or side-band ripple, have been attempted with thin-film interference filters by assembling combinations of layers having varying optical thicknesses. For example, U.S. Pat. No. 6,157,490 to Wheatley et al. discloses a multi-layer film in which one of a pair of alternating material layers progressively varies in thickness to sharpen band-edges on one or both sides of reflection bands. Although such thickness variations can be demonstrated mathematically to produce sharper spectral responses, achieving the desired thickness variations in practice remains problematic as the inherent self-correcting effects of xe2x80x9cturning point monitoringxe2x80x9d are lost.
Generalized patterns of ultra-thin films have been used to produce antireflective coatings exhibiting gradient index profiles. For example, U.S. Pat. No. 4,666,250 to Southwell discloses a generalized arrangement of ultra-thin layers in which a combination of layers having equal physical or optical thickness but differing between two refractive indices are arranged in different orders by trial and error to approximate a predetermined gradient index profile. The ultra-thin layers in the range of ten nanometers or less (which is much shorter than the near micrometer range wavelengths of intended use) are thin enough to exhibit in combination progressively varying refractive indices with low dispersion over an intended spectral range.
Southwell""s trial-and-error approach to reaching a desired spectral response starts with a particular arrangement of high and low index layers, reverses one layer at a time (i.e., changes a layer from low to high index or from high to low index), and evaluates the result using a merit function. If the changed response more closely matches the desired response, the change is maintained. Otherwise, the change is reversed. The remaining layers are similarly tested in a prescribed sequence. Good results of this approach can also be demonstrated mathematically, but the ultra-thin layers are too thin for conventional monitoring beams.
Although the xe2x80x9cturning point monitoringxe2x80x9d technique provides significant advantages for accurately monitoring the thicknesses of vacuum deposited layers, the layer thicknesses to which the technique can be used are limited to multiples of quarter-wavelength travels of monitoring light beams. Filter designs, such as those proposed for sharper band responses requiring progressive variations in layer thickness, and filter designs, such as those proposed for anti-reflective coatings requiring ultra-thin layers, both depart from quarter-wavelength optical thicknesses and fail to benefit from the xe2x80x9cturning point monitoringxe2x80x9d technique.
More complex filter responses, such as required for gain-flattening filters, have been even more out of reach of the benefits of xe2x80x9cturning point monitoringxe2x80x9d techniques. Gain-flattening thin-film filter designs have required nearly arbitrary variations in the thicknesses of the deposited layers throughout the thin-film filter structures to equalize the gain spectrum of optical amplifiers. Although mathematically feasible, such gain-flattening designs are not amenable to manufacture. Random errors in layer thicknesses of even thousandths of a percent can lead to significant equalization errors.
Our invention extends the benefits of xe2x80x9cturning point monitoringxe2x80x9d techniques to the manufacture of gain-flattening thin-film interference filters for fiber optic communication systems. Thin-film unit sub-layers having physical thicknesses equal to a quarter wavelength travel of a monitoring beam are arranged in a generalized pattern for producing a complex spectral response capable of adjusting power among a plurality of different wavelength channels. The complex spectral responses are achieved to required accuracy by exploiting the self-correcting effects of xe2x80x9cturning point monitoringxe2x80x9d throughout the deposition of the more generalized pattern of quarter-wavelength thickness unit sub-layers.
One example of our invention is a gain-flattening interference filter having a plurality of layers separately formed from one of at least two different refractive index materials. Each of the layers is composed of an integer multiple number of unit sub-layers having a common refractive index and a common physical thickness corresponding to a quarter-wavelength travel of a monitoring light beam as transmitted through the unit sub-layers. The different refractive index materials include high and low refractive index materials arranged in the unit sub-layers that differ in physical thicknesses in inverse ratio to their refractive indices. The high and low index unit sub-layers are arranged in a substantially non-repeating pattern for producing different attenuating effects among different wavelength channels conveyed through the filter.
The monitoring light beam, which determines the quarter-wavelength travel through the sub-layers, has a wavelength that can differ from a central wavelength of the different wavelength channels attenuated by the filter. Ordinarily, the wavelength of the monitoring light beam is expected to be beyond a working range of wavelengths spanned by the different wavelength channels attenuated by the filter. Monitoring beam wavelengths longer than the working range of the different wavelength channels have been found to produce some of the better designs.
Our gain-flattening interference filter can be made by vacuum depositing layers of thin films exhibiting different refractive indices. Each of the layers is composed of one or more unit sub-layers having physical thicknesses equal to a quarter wavelength travel of a monitoring light beam. The unit sub-layers are arranged in a generalized non-repeating pattern. Filter performance is optimized by exploiting design freedoms that include variations in the monitoring beam wavelength, the refractive indices of the different index materials, and the number and order of the unit sub-layers.
The exemplary filter design begins with a determination of the optical power adjustments required for a plurality of different wavelength channels. At least two different refractive index materials capable of being separately deposited as high and low refractive index layers are chosen along with a wavelength for the monitoring light beam. The high and low refractive index layers are deposited as integer multiples of unit sub-layers having physical thicknesses corresponding to quarter-wavelength travels of the monitoring light beam. The unit sub-layers of the high and low refractive index layers are arranged in a substantially non-repeating pattern for producing different attenuating effects among the different wavelength channels conveyed through the filter.
During the deposition process, the monitoring beam is transmitted through the high and low index layers, and transmissions of the monitoring beam are measured to identify transitions associated with deposition thicknesses corresponding to integer multiples of the quarter-wavelength travels of the monitoring beam. The wavelength for the monitoring beam is preferably chosen from beyond a working range of wavelengths spanned by the different wavelength channels attenuated by the filter.
Combinations of the high and low index unit sub-layers can be organized into a set of base permutations including alternatives that vary in both number and order of the high and low index unit sub-layers. Each of a determined number of levels is filled by one of the base permutations. Collective optical performance characteristics of the levels are evaluated and compared to desired relative optical power adjustments among the plurality of different wavelength channels. The levels are refilled from the set of base permutations based on the results of the comparison for better matching the collective optical performance characteristics of the levels with the desired optical power adjustments among the plurality of different wavelength channels.
The refilling and collective re-evaluation of the levels are repeated in an iterative manner to converge the collective optical performance characteristics of the filled levels toward the desired optical power adjustments among the plurality of different wavelength channels. The performance characteristics of the base permutations can be pre-evaluated for simplifying the re-evaluation of the collective optical performance characteristics of the multiple levels.
Selected filter designs corresponding to particular iterations of the filled levels can be saved based on criteria for matching the determined optical power adjustments among the plurality of different wavelength channels. Additional criteria can be used for choosing among the saved filter designs. For example, the additional criteria can include preferences for fewer total number of the high and low index layers, for monitoring wavelengths maximizing signal-to-noise ratio, and for ease of manufacture. Designs containing insensitive layers that exhibit only weak transmission extrema at quarter-wavelength thicknesses can be excluded.